1. Technical Field
The techniques described herein relate to semiconductor light-emitting structures capable of emitting light in the yellow-green portion of the visible electromagnetic spectrum, substrates on which such structures may be formed, and methods for fabricating such structures.
2. Discussion of Related Art
Currently, light-emitting semiconductor technology lacks a commercially viable materials system for green emission, as no green group III-V semiconductor lasers with pure emission (e.g., not frequency doubled) are known to have been previously demonstrated. (As used herein, “green” emission refers to the yellow-green portion of the visible electromagnetic spectrum). Light-emitting devices including aluminum in their active region, such as (Al)InGaN-based light-emitting devices (LEDs) on small lattice constant substrates (e.g., sapphire) and AlInGaP LEDs on GaAs substrates (e.g., epitaxially grown on GaAs substrates), lack the efficiency of other emitters.
An efficient yellow-green or green emitter with a comparable efficiency to blue and red emitters does not yet exist. Materials that currently dominate the LED market have not demonstrated very good performance in the yellow-green part of the spectrum, as illustrated in FIG. 1. This problem is colloquially known as the “green gap.” These wavelengths are significant because displays that emit yellow-green light appear much brighter than other colors for a given power output, because the human eye is most sensitive to the yellow-green portion of the visible light spectrum. As illustrated in FIG. 1, previous devices emitting in the yellow-green wavelength range of about 500-600 nm do not approach the 10%-20% power conversion efficiencies available at longer and shorter wavelengths.
In the GaN material system, green emission can be achieved through the use of InGaN quantum well or quantum dot layers. However, despite the very short minority carrier lifetimes induced by the quantum wells, the high dislocation density present in GaN layers combined with the inability to add enough indium (In) to form deep quantum wells (due to InGaN—GaN lattice-mismatch and thermodynamic constraints) can result in lower efficiency as compared with typical blue emission performance.
Another material system that has been considered for yellow-green emission is AlInGaP. Compositions of AlInGaP with a direct band gap can be used as the active material of a light-emitting device such as a laser diode or light-emitting diode (LED). Previously, compositions of AlInGaP that are lattice matched to GaAs (having a lattice constant of 0.5565325 nm) have been used to produce red to green light emitters. As the emitted wavelength becomes smaller (e.g., greener, rather than redder), larger band gap active regions must be used, reducing the available electronic and optical confinement in the devices and thus making them less efficient.
Green-emitting AlInGaP devices that are lattice-matched to GaAs have poor internal quantum efficiency due to the proximity of the indirect-direct bandgap crossover at the compositions of interest, as well as due to oxygen-related defects. Some of the shorter wavelength AlInGaP devices with at least minimal brightness operate in the range of 500 to 600 nm. These 500-600 nm devices often have very poor color purity, leading to an undersaturated appearance. This is a particular problem in the green region, where a deviation of as little as 2 nm may be discernable to the human eye.
The prediction of the electronic structure of AlInGaP alloys is quite complex. Little agreement exists in the literature regarding electronic parameters such as deformation potentials and band offsets. Even the exact values of Γ and X band gaps and the compositions of alloys at which indirect-direct crossovers occur are contested. Especially with compositions of the alloy far away from the lattice constant of GaAs, it is currently difficult to accurately predict the electronic structure of light-emitting designs.
A need therefore exists for semiconductor light-emitting structures capable of efficiently emitting light in the yellow-green portion of the electromagnetic spectrum.